In wireless communication systems, antenna diversity plays an important role in increasing the system link robustness. OFDM is used as a modulation technique for transmitting digital data using radio frequency signals (RF). In OFDM, a radio signal is divided into multiple sub-signals that are transmitted simultaneously at different frequencies to a receiver. Each sub-signal travels within its own unique frequency range (sub-channel), which is modulated by the data. OFDM distributes the data over multiple channels, spaced apart at different frequencies.
OFDM modulation is typically performed using a transform such as Fast Fourier Transform (FFT) process wherein bits of data are encoded in the frequency-domain onto sub-channels. As such, in the transmitter, an Inverse FFT (IFFT) is performed on the set of frequency channels to generate a time-domain OFDM symbol for transmission over a communication channel. The IFFT process converts the frequency-domain phase and amplitude data for each sub-channel into a block of time-domain samples which are converted to an analogue modulating signal for an RF modulator. In the receiver, the OFDM signals are processed by performing an FFT process on each symbol to convert the time-domain data into frequency-domain data, and the data is then decoded by examining the phase and amplitude of the sub-channels. Therefore, at the receiver the reverse process of the transmitter is implemented. Further, transmit antenna diversity schemes are used to improve the OFDM system reliability. Such transmit diversity schemes in OFDM systems are encoded in the frequency-domain as described.
OFDM has been selected as the basis for the high speed wireless local area network (WLAN) standards by the IEEE standardization group. Referring to FIG. 1, a conventional transmitter 100 for an OFDM MIMO system implementing WLAN comprises a source of data bits 102, a spatial parser 104, and multiple data stream processing paths 106. Each data stream processing path 106 comprises: a channel encoder & puncturer 108, a frequency interleaver 110, a constellation mapper 112, an IFFT function 114, a guard-interval (GI) window insertion 116 and an RF modulator 118.
The system diagram in FIG. 1 represents a MIMO OFDM structure for 20 MHz channelization, and uses two independent convolutional-code encoders for the two data paths. Further, two IEEE 802.11a interleavers are used independently, each interleaver 110 corresponding to each encoder. Interleaver in FIG. 1 provides an optimal design for single antenna systems by fully exploring the frequency diversity. However, for multiple antenna systems, this design does not explore the spatial diversity brought in by the multiple antennas. Thus, there is a need for an interleaver design to fully explore the diversity of the MIMO OFDM systems.